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A TEM-APFIM INVESTIGATION OF BORON-DOPED Ni3Al
J. Horton, M. Miller
To cite this version:
J. Horton, M. Miller. A TEM-APFIM INVESTIGATION OF BORON-DOPED Ni3Al. Journal de
Physique Colloques, 1986, 47 (C2), pp.C2-209-C2-214. �10.1051/jphyscol:1986231�. �jpa-00225664�
JOURNAL DE PHYSIQUE
Colloque C2, suppl6ment au n03, Tome 47, mars 1986 page c z - 2 0 9
A TEM-APFIM INVESTIGATION OF BORON-DOPED Ni3A1
J.A. HORTON and M.K. MILLER
Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, U.S.A.
A b s t r a c t - Boron a d d i t i o n s t o n o r m a l l y b r i t t l e Ni3A1 r e s u l t i n room t e m p e r a t u r e d u c t i l i t i e s o v e r 50%. L i t t l e i s p r e s e n t l y known about t h e r o l e o f boron i n s t r e n g t h e n i n g t h e o t h e r w i s e b r i t t l e g r a i n boundaries. A TEM s t r u c t u r a l charac- t e r i z a t i o n o f c a s t m a t e r i a l shows a f e a t u r e l e s s m i c r o s t r u c t u r e w h i l e r a p i d l y s o l i - d i f i e d m a t e r i a l has s u b s t a n t i a l a1 uminum enrichments c o u p l e d w i t h a bimodal APB d i s t r i b u t i o n . P r e l i m i n a r y atom probe i n v e s t i g a t i o n s o f c a s t m a t e r i a l f o u n d t h e boron i n s o l u t i o n , w i t h no evidence o f c l u s t e r i n g i n t h e g r a i n i n t e r i o r s . A s m a l l enrichment o f boron was found a t t w i n boundaries.
I - INTRODUCTION
Ni3A1 i s an o r d e r e d i n t e r m e t a l l i c compound w i t h good h i g h t e m p e r a t u r e s t r e n g t h and o x i d a t i o n r e s i s t a n c e . These p r o p e r t i e s make t h i s m a t e r i a l a t t r a c t i v e f o r h i g h tem- p e r a t u r e s t r u c t u r a l a p p l i c a t i o n s . P o l y c r y s t a l l i n e N i 3A1 has low d u c t i l i t y and f r a c t u r e s i n t e r g r a n u l a r l y , a p p a r e n t l y due t o an i n h e r e n t g r a i n boundary weakness.
Aoki and Izumi (1) f o u n d t h a t small a d d i t i o n s o f boron r a i s e t h e room t e m p e r a t u r e t e n s i l e d u c t i l i t i e s t o 35%. L i u and Koch (2) f u r t h e r r e f i n e d t h e parameters f o r boron c o n c e n t r a t i o n and thermomechanical t r e a t m e n t t o produce t e n s i l e d u c t i l i t i e s o f g r e a t e r t h a n 50% f o r a l l o y s w i t h s u b s t o i c h i o m e t r i c aluminum c o n t e n t s .
Auger spectroscopy o f i n t e r g r a n u l a r f r a c t u r e s u r f a c e s i n N i 3A1 have n o t f o u n d an e m b r i t t l i n g agent a l t h o u g h s u l f u r i s s l i g h t l y segregated t o t h e boundaries (3,4,5).
These i n v e s t i g a t i o n s i n d i c a t e d t h a t t h e g r a i n boundaries a r e i n t r i n s i c a l l y b r i t t l e . I t has been shown t h a t boron segregates t o grain boundaries b u t n o t t o f r e e s u r - f a c e s w h i l e s u l f u r segregates more s t r o n g l y t o f r e e s u r f a c e s t h a n t o boundaries (3,5). Auger spectroscopy i s o n l y a b l e t o a n a l y z e g r a i n boundaries which f a i l i n t e r g r a n u l a r l y and t h e r e f o r e t h e s e a r e p r o b a b l y n o t t h e s t r o n g e s t boundaries. An atom probe f i e l d i o n microscope (APFIM) i n v e s t i g a t i o n , however p r o v i d e s a t e c h n i q u e capable o f a n a l y z i n g t h e boron d i s t r i b u t i o n and s t r u c t u r e o f t h e good boundaries t h a t would n o t f a i l i n t e r g r a n u l a r l y .
Concurrent w i t h a program t o develop c a s t a b l e a l l o y s based on Ni3A1 + B, an a l t e r - n a t i v e p r o c e s s i n g r o u t e u t i l i z i n g r a p i d s o l i d i f i c a t i o n i s a l s o under development.
The d u c t i l i z i n g e f f e c t o f boron i s r e t a i n e d under most r a p i d s o l i d i f i c a t i o n con- d i t i o n s (6,7). Rapid s o l i d i f i c a t i o n o f p u r e Ni3A1 does n o t r e s u l t i n a d u c t i l e m a t e r i a l , w h i l e t h e f a s t e s t r a t e s o f s o l i d i f i c a t i o n o f b o r o n - m o d i f i e d Ni3A1 showed l e s s d u c t i l i t y t h a n c a s t m a t e r i a l (6). These r e s u l t s suggest t h a t boron has a s i m i l a r e f f e c t on t h e . g r a i n boundaries i n r a p i d l y s o l i d i f i e d Ni3A1 as i n c a s t m a t e r i a l . The m i c r o s t r u c t u r e s o f r a p i d l y s o l i d i f i e d Ni 3A1 have been c h a r a c t e r i z e d by t r a n s m i s s i o n e l e c t r o n microscopy (TEM) a f t e r r a p i d s o l i d i f i c a t i o n by arc-hammer
(8), m e l t s p i n n i n g ( Y ) , and powder p r o c e s s i n g (10). An i s o t r o p i c a n t i p h a s e bound-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986231
~ 2 - 2 1 0 JOURNAL DE PHYSIQUE
a r y (APB) network w i t h a bimodal s i z e d i s t r i b u t i o n and s u b s t a n t i a l a l u m i n u r ~ ~ enrichments (8) i s p r e s e n t a f t e r r a p i d s o l i d i f i c a t i o n . H e a v i l y t w i n n e d s t r u c t u r e s w i t h a m a r t e n s i t i c appearance a r e sometimes p r e s e n t . T h i s paper r e p o r t s TEM c h a r a c t e r i z a t i o n and p r e l i m i n a r y APFIM i n v e s t i g a t i o n o f b o r o n - m o d i f i e d Ni3A1.
I1 - EXPERIMENTAL PROCEDURE
A l l o y s o f Ni3A1 were p r e p a r e d by a r c m e l t i n g and d r o p c a s t i n g (3). T h i s process produces a f i n e i n g o t g r a i n s t r u c t u r e and m i n i m i z e s c o m p o s i t i o n a l s e g r e g a t i o n d u r i n g s o l i d i f i c a t i o n . The i n g o t s were homogenized f o r 5 h a t 1050°C and t h e n f a b r i c a t e d i n t o s h e e t s by r e p e a t e d c o l d r o l l i ng w i t h i n t e r m e d i a t e anneals a t 1000°C. Atom probe specimens were produced f r o m a 0.5 mm d i a m e t e r w i r e made by r e p e a t e d swaging (15%) o f an i n g o t w i t h i n t e r m e d i a t e 300 s anneals a t 1000°C.
Rapid s o l i d i f i c a t i o n specimens were produced e i t h e r i n an arc-hammer apparatus ( 6 ) o r were m e l t spun. Chemical a n a l y s i s o f t h e base a l l o y w i t h nominal c o m p o s i t i o n o f 24 at. % A1 and 0.24 % B (500 wppm) y i e l d e d 75.6 a t . % Ni, 24.1% A l , 0.23% B, 0.049% 0, 0.018% C and l e s s t h a n 0.0005% o f Hf, Zr, Cu, Co, Fe, Mn, Ca, S, P, S i , Mg, Au, and Ce. A l l c o m p o s i t i o n s a r e quoted i n a t o m i c p e r c e n t . TEM examinations were performed u s i n g a P h i l i p s EM400T equipped w i t h a f i e l d e m i s s i o n gun and an EDAX energy d i s p e r s i v e s p e c t r o m e t e r (EDS). Atom probe f i e l d - i o n e x a ~ n i n a t i ons were performed a t t h e US S t e e l Research Lab a t M o n r o e v i l l e , PA.
I 1 1 - RESULTS AND DISCUSSION
TEM m i c r o s t r u c t u r a l c h a r a c t e r i z a t i o n o f t h e thermomechanically processed and homog- e n i z e d base Ni 3A1 a l l o y shows a f u l l y ordered, s i n g l e phase, c h e m i c a l l y homogeneous s t r u c t u r e . No e v i d e n c e o f p r e c i p i t a t e s i n t h e m a t r i x n o r a t t h e g r a i n boundaries was o b t a i n e d b y e l e c t r o n d i f f r a c t i o n , d i f f r a c t i o n c o n t r a s t imaging, o r l a t t i c e imaging. These t e c h n i q u e s may n o t d e t e c t p r e c i p i t a t e s s m a l l e r t h a n a p p r o x i m a t e l y 2 nm. EDS a n a l y s i s showed no elemental s e g r e g a t i o n e i t h e r i n t h e m a t r i x o r i n t h e v i c i n i t y o f t h e g r a i n boundaries. I n a l l o y s w i t h boron c o n t e n t above t h e s o l u b i - l i t y l i m i t (-1.0% B) p r e c i p i t a t e s a r e formed as shown i n F i g u r e 1 i n a Ni-24A1-28 a1 1 oy. E l e c t r o n d i f f r a c t i o n p a t t e r n s and an EDS a n a l y s i s o f t h e s e p r e c i p i t a t e s y i e l d e d a metal c o n t e n t o f 86% N i and 14% A1 which agrees w i t h t h e c o m p o s i t i o n and s t r u c t u r e o f t h e known n i c k e l aluminum b o r i d e t a u phase, Ni20A13B6.
H i g h q u a l i t y f i e l d i o n micrographs (FIM) as shown i n F i g u r e 2 c o u l d be o b t a i n e d , d e s p i t e t h e s u b s t o i c h i o m e t r i c aluminum c o n t e n t and t h e boron a d d i t i o n . The L12 o r d e r e d s t r u c t u r e was observed i n t h e a l t e r n a t i n g c o n t r a s t o f s e q u e n t i a l 001 p l a n e s which a l t e r n a t e l y have p u r e N i and mixed Ni+Al c o n t e n t , as shown i n F i g u r e 3. W i t h s u i t a b l e e x p e r i m e n t a l c o n d i t i o n s , an atom probe chemical a n a l y s i s o f t h e homoge- neous m a t r i x o f t h e n o r m a l l y c a s t m a t e r i a l gave a v a l u e o f 23.8 + 1.0% A1 as com- p a r e d t o a chemical a n a l y s i s o f 24.1%. Most o f t h e boron was found t o remain i n s o l i d s o l u t i o n i n t h e m a t r i x . No evidence o f widespread c l u s t e r i n g o f t h e boron was observed i n agreement w i t h r e c e n t p o s i t r o n a n n i h i l a t i o n r e s u l t s i n a s i m i l a r m a t e r i a l (12). A f i e l d - i o n micrograph o f a specimen c o n t a i n i n g a t w i n boundary i n t h e base a l l o y i s shown i n F i g u r e 4. The t r a c e o f t h e boundary d u r i n g f i e l d evapo- r a t i o n sequence r e v e a l e d s e v e r a l d i s c o n t i n u i t i e s i n t h e boundary plane. A s m a l l i n c r e a s e i n t h e boron l e v e l was measured a t t h e boundary.
R a p i d s o l i d i f i c a t i o n o f Ni3A1 r e s u l t s i n a complex s t r u c t u r e o f a n t i p h a s e bound- a r i e s and c o m p o s i t i o n a l inhomogeneities. Normally, N i 3A1 c o n t a i n s no APBs because t h e o r d e r e d phase extends p r a c t i c a l l y t o t h e m e l t i n g p o i n t f o r t h i s c o m p o s i t i o n as shown i n F i g u r e 5. However, r a p i d s o l i d i f i c a t i o n w i t h c o m p o s i t i o n s on t h e n i c k e l r i c h s i d e o f t h e e u t e c t i c r e s u l t s i n s o l i d i f i c a t i o n o f r e g i o n s w i t h 1 t o 2% e n r i c h - ment of n i c k e l . These r e g i o n s a r e i n i t i a l l y i n t h e d i s o r d e r e d a phase f i e l d and t h e n c r o s s i n t o t h e c o m p l e t e l y o r d e r e d ' a phase f i e l d a t a t e m p e r a t u r e s u f f i c i e n t l y l o w t o r e s u l t i n an APB network w i t h a s m a l l domain s i z e ( a p p r o x i m a t e l y 50 nm).
The a d j a c e n t r e g i o n s , which a r e aluminum e n r i c h e d , o r d e r a t a much h i g h e r tem-
p e r a t u r e and t h e r e f o r e have a l a r g e r domain s i z e . An example o f t h i s s t r u c t u r e and
a n EDS l i n e scan across t h e i n d i c a t e d r e g i o n i s shown i n F i g u r e 6. I n f i n e r g r a i n m a t e r i a l t h e s m a l l -domain n i c k e l - e n r i c h e d r e g i o n s were t y p i c a l l y i n t h e c e n t e r o f t h e g r a i n w i t h large-domain aluminum-enriched r e g i o n s a d j a c e n t t o t h e g r a i n bound- ary. I n a few cases t h e s m a l l domain r e g i o n extended t o a g r a i n boundary. I n c o a r s e g r a i n e d d e n d r i t i c m a t e r i a l such as produced a f t e r w e l d i n g where t h e
F i g . 1 - TEM m i c r o g r a p h and d i f f r a c t i o n Fig. 2 - Neon FIM m i c r o g r a p h a t 30K p a t t e r n o f Ni-24 A1-2 B showing t a u showing a h i g h q u a l i t y image o f sub- phase p r e c i p i t a t e s . s t o i c h i o m e t r i c Ni-24.0 A1-0.24 B.
F i g . 3 - Neon FIM micrographs a t 78K showing t h e a l t e r n a t i n g mixed Ni+Al and p u r e N i [001] p l a n e s o f t h e L12 o r d e r e d s t r u c t u r e as d i m and b r i g h t r i n g s .
s o l i d f i c a t i o n r a t e i s i n t e r m e d i a t e between c a s t i n g and r a p i d s o l i d i f i c a t i o n , s e v e r a l s m a l l domain r e g i o n s p e r g r a i n a r e t y p i c a l .
Rapid s o l i d i f i c a t i o n of Ni3A1 w i t h c o m p o s i t i o n s on t h e aluminum r i c h s i d e of t h e
e u t e c t i c a l s o r e s u l t e d i n a s m a l l g r a i n e d m a t e r i a l w i t h a complex s t r u c t u r e ,
F i g u r e 7. Many o f t h e g r a i n s were i n t e r n a l l y t w i n n e d w i t h an o v e r a l l m a r t e n s i t i c
appearance. O t h e r g r a i n s were u n t r a n s f o r m e d b u t had an APB s t r u c t u r e s i m i l a r t o
t h a t d e s c r i b e d above. No c o m p o s i t i o n a l d i f f e r e n c e s were found by EDS a n a l y s i s o f
t r a n s f o r m e d and untransformed g r a i n s .
C2-212 JOURNAL DE PHYSIQUE
F i g . 4 - Neon FIM mic;ograph o f a A1-0.24 B.
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